ACT Science: Molarity and Concentration—Master Dilution Calculations

Molarity (M) = moles of solute / liters of solution. A 1 M solution contains 1 mole of dissolved substance per liter. When you dilute a solution, you add solvent (usually water) to decrease concentration. The key principle: moles of solute stay the same, but volume increases, so molarity decreases. The dilution equation M₁V₁=M₂V₂ tracks this: M₁ (initial molarity) × V₁ (initial volume) = M₂ (final molarity) × V₂ (final volume). The left side (moles before dilution) equals the right side (moles after dilution). Dilution is the most commonly tested molarity concept on the ACT because it requires both conceptual understanding and formula application.

Example: You have 100 mL of 2 M HCl. You dilute it to 500 mL. What is the final molarity? M₁V₁=M₂V₂ → 2×100=M₂×500 → 200=500M₂ → M₂=0.4 M.

Scenario 1 (Find final molarity): You have 250 mL of 3 M NaCl. You dilute to 750 mL. M₁V₁=M₂V₂ → 3×250=M₂×750 → 750=750M₂ → M₂=1 M. Scenario 2 (Find final volume): You have 50 mL of 4 M KOH. You want to dilute it to 1 M. What volume is needed? 4×50=1×V₂ → 200=V₂. Final volume is 200 mL. Scenario 3 (Find initial molarity): You dilute 100 mL of unknown molarity to 400 mL, resulting in 0.5 M. M₁×100=0.5×400 → M₁×100=200 → M₁=2 M. Know all three forms of the dilution equation: M₂=M₁V₁/V₂, V₂=M₁V₁/M₂, M₁=M₂V₂/V₁.

Solve these three scenarios daily until you complete each in under 90 seconds.

Problem 1: Calculate molarity of a solution containing 0.5 moles of HCl in 2 liters. Answer: M=0.5/2=0.25 M. Problem 2: You have 200 mL of 0.8 M H₂SO₄. You dilute to 1000 mL. Find new molarity. M₁V₁=M₂V₂ → 0.8×200=M₂×1000 → M₂=0.16 M. Problem 3: You need 100 mL of 0.5 M NaOH, but you only have 2 M NaOH stock solution. What volume of stock is needed? 2×V₁=0.5×100 → V₁=25 mL. Problem 4: A 0.3 M solution of unknown volume is diluted to 500 mL, giving 0.1 M. Find initial volume. 0.3×V₁=0.1×500 → V₁=167 mL. Problem 5: How many moles are in 250 mL of 0.4 M KCl? moles=M×V=0.4×0.25=0.1 moles. Complete all five daily until speed and accuracy are automatic.

On test day, write the dilution formula and three rearranged forms on your scratch paper immediately.

Molarity and dilution questions appear in 1-2 ACT Science passages (chemistry section). They are purely computational once you know the formulas and understand the concepts. Investing 20 minutes in this topic yields 1-2 guaranteed points because the math is mechanical and the formula is always provided or easy to recall.

Master this concept two days before the test and review formula rearrangement one day before. You will answer molarity questions with confidence.